Air will leak through a building envelope that is not well sealed. This leakage of air decreases the comfort of a residence by allowing moisture, cold drafts, and unwanted noise to enter and may lower indoor air quality by allowing in dust and airborne pollutants. In addition, air leakage accounts for between 25 percent and 40 percent of the energy used for heating and cooling in a typical residence.
The amount of air leakage in a house depends on two factors. The first is the number and size of air leakage paths through the building envelope. As shown in Figure 1, these paths include joints between building materials, gaps around doors and windows, and penetration for piping, wiring and ducts. The second factor is the difference in air pressure between the inside and outside. Pressure differences are caused by wind, indoor and outdoor temperature differences (stack effect), chimney and flue exhaust fans, equipment with exhaust fans (dryers, central vacuums) and ventilation fans (bath, kitchen).
Air sealing the building envelope is one of the most critical features of an energy efficient home. To prevent air leakage, it is best to seal the building envelope during construction prior to installation of the drywall. Once covered, many air leakage paths will be more difficult and costly to access and properly seal. A "blower door" test (typically included with a Home Energy Rating) is a good way to identify air leakage paths so that they can be sealed using an appropriate material. There are many products available for air sealing including caulks, foams, weatherstripping, gaskets, and door sweeps.
In new homes that are tightly sealed, you will want to make sure there is adequate fresh air for ventilation. It is better to use controlled or active ventilation than to rely on air leakage. In many ENERGY STAR qualified homes, an active ventilation system is installed along with air sealing to ensure that fresh air is provided.